Pb-In-Sn tall C-4 for fatigue enhancement

ABSTRACT

A solder column structure particularly useful for joining electronic components by C-4 interconnection is provided comprising a solder column attached at one end to one of the substrates being joined and having a layer of indium at the other end. During reflow, to join the other substrate, the indium melts with part of the solder column forming a Pb--Sn--In ternary alloy joint having enhanced fatigue resistance. A method for using the solder column to make electronic component assemblies and electronic component assemblies made using the method and solder column are also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a solder structure in the form of a columnwhich structure provides enhanced fatigue life properties when reflowedand used to bond substrates and, more particularly, to a solderstructure having a metal portion at one end thereof which forms an alloywith solder and which structure is used to make C-4 connections inelectronic components such as joining a chip to a substrate.

2. Description of Related Art

The use of solder to join materials such as components of an electronicstructure is well known in the art. In the electronics area there are amyriad of electronic components which require connection to otherelectronic components or to other levels of packaging. Examples includemounting of integrated circuit chips to a metallized substrate,multilayer ceramic substrate, laminate organic substrate, glass ceramicsubstrate, card (direct-chip-attach, DCA), and any substrate made ofcomposite materials meeting thermal and mechanical properties. For thesake of clarity and consistency in describing the present invention thespecification will be directed to electronic components made usingControlled Collapse Chip Connection (C-4) technology and, in particular,to the use of C-4 solder columns as compared with spherical solderbumps.

C-4 technology is an interconnection technology developed by IBM as analternative to wire bonding. Broadly stated, in one application, one ormore integrated circuit chips are mounted above a single or multilayersubstrate and pads on the chip are electrically connected tocorresponding pads on the substrate by a plurality of electricalconnections known as solder bumps or solder columns. An example of a C-4configuration is a square grid array which is 11 C-4 pads long by 11 C-4pads wide on 10 mil centers. A five mil solder bump is located at everyintersection in the grid except one which is typically displaced fororientation purposes. A popular chip is a circuit "computer-on-a-chip"which has 762 C-4 solder bumps in a 29×29 area array.

The C-4 technology has also extended to other applications and is nowused on thin-film resistor and composite chips in hybrid modularapplications. Solder pads for this application are very large-about 25mil in diameter. At the other extreme, C-4s have been used for precisionregistration and alignment in the joining of a GaAs wave guide. The mostdense area array reported has been a 128×128 array of 1 mil bumps onabout 2 mil centers resulting in 16,000 pads.

The C-4 technology typically utilizes spherical solder bumps depositedon solder wettable metal terminals on the chip and a matching foot printof solder wettable terminals on the substrate to be joined thereto. Theupside-down chip (flip chip) is aligned to the substrate, and all jointsare made simultaneously by reflowing the solder bumps. The flow on thechip is limited by a ball limiting metallurgy (BLM) pad which isgenerally a circular pad of evaporated, thin-film metal such aschromium, copper and gold that provides the sealing of the via as wellas the solderable, conductive base for the solder bump. A very thickdeposit of evaporated solder acts as the primary conduction and joiningmaterial between chip and substrate.

Melting point has been a consideration in the choice of solder alloysfor C-4s. Lead solders, especially 95 Pb/5 Sn have been widely used withalumina ceramic substrates because of their high melting point ofapproximately 315° C. Their use for the chip connection allows otherlower-melting point solders to be used at the module-to-card orcard-to-board packaging level without remelting the chip's C-4s.Intermediate melting point solders such as eutectic 63 Sn/37 Pb (meltingpoint 183° C.) and a 50 Pb/50In melting point of approximately 220° C.have been used. In "Microelectronics Packaging Handbook", edited by R.R. Tummala and Rymaszewski, 1989, van Nostrand Reinhold, pages 361-391,C-4 chip to package interconnections as well as typical solders used inC-4 technology are discussed and this reference is hereby incorporatedby reference.

Once the solder bumps or columns are deposited on the BLM, the joiningof the BLM of the chips to the substrate using C-4 technology isrelatively straight forward. Flux, either water-white rosin forhigh-lead solders with water-soluble flux for low lead and otherlow-melting solders, is normally placed on the substrate to be joined asa temporary adhesive to hold the chips in place. Such an assembly isthen subjected to a reflow thermal cycle wherein the pads on the chipand the pads on the substrate self-align due to the high-surface-tensionforces of the solder to complete the assembly. Once the chip-joiningoperation is complete, cleaning of flux residues is accomplished withsuch solvents as chlorinated solvents or xylene. The assembly is thenelectrically tested.

New technologies are continuously increasing the number of C-4interconnections per device, and/or the size of the chip, both of whichaffect the stresses on the solder interconnections. As chips become moreand more dense, higher input/output counts will drive area arrays ofterminals to as many as 155,000 pads on a 20 mm chip. This will resultin the number of pads increasing while the pad sizes and spacingsdecrease. The new technologies will induce large strains to the solderjoint and new solders and solder structures are needed to meet thefatigue requirements of these types interconnections.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide a solder andsolder structure which provides enhanced fatigue life properties whenused to bond substrates particularly electronic substrates such as achip to a multilayer ceramic substrate.

It is another object of the present invention to provide a method formaking C-4 solder interconnections, using the specially defined columnsolder structure of the invention.

A further object of the invention is to provide C-4 containingelectronic structures made using the solder column structure and methodof the invention.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects, which will be apparent to those skilled inthe art, are achieved by the present invention which relates in a firstaspect to a solder structure preferably in the form of a column which,when used to bond electronic substrates together, forms an enhancedfatigue resistant solder bond the solder structure comprising a soldercolumn attached at one end to pads or other bonding sites on one of theelectronic substrates to be joined and having at the other end a layerof metal which forms a ternary alloy with the solder (preferably singlephase) when the solder column is reflowed to join the other substrate.The metal layer is preferably indium.

It should be appreciated that the reflow temperature is preferably belowthe melting point of the solder and that the solder in contact with themetal layer melts at such a lower temperature due to the formation of aternary alloy. The solder structure may be a solder column up to about15 mils tall or higher, typically up to about 10 mils tall and having anindium (metal) thickness of up to about 1.5 mils or higher. A preferredsolder column has a solder height of about 9 to 15 mils and a metallayer height of about 0.5 to 2 mils. The solder preferably comprises upto 3% by weight tin and the balance lead. The height of the column maybe over three times its diameter (the ratio of height to diametertypically being termed the aspect ratio) with high aspect ratiosproviding enhanced fatigue resistance. In general, a C-4 column having aheight over three (3) times its diameter will roughly have approximatelytwenty-five (25) times the fatigue life of a standard C-4 column havinga 0.6 aspect ratio.

In another aspect of the present invention, a method is provided formaking C-4 solder electrical interconnections in an electronic componentassembly comprising the steps of:

applying solder to metallized pads on the surface of a first substrateof the electronic component and forming a solder structure preferably acolumn structure which structure is bonded to the pads;

forming a layer of metal, preferably indium, on the non-bonded end ofthe solder structure which metal melts during a reflow cycle to allowthe column to bond pads on a second substrate to the corresponding padson the first substrate and which metal forms a ternary alloy with thesolder of the solder structure;

positioning the pads on the surface of the second substrate of theelectronic component assembly to be joined proximate to the ends of thesolder columns of the corresponding pads on the first substrate;

heating the substrates to a temperature sufficient to melt the indiumand part of the solder column forming a solder-indium ternary alloy(Pb--Sn--In) and alloy bond between the solder structure and the pads onthe second substrate; and

cooling the bonded interconnected assembly.

In a further aspect of the invention, electronic assemblies made by theabove method are also provided with the electronic assemblies comprisingelectronic components joined together, the components including amultilayer ceramic substrate and a semiconductor chip.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIGS. 1A-1E show a method for forming a solder structure of theinvention and using the solder structure to bond a pad on a chip to apad on a substrate.

FIG. 2 is a graph showing the Pb--Sn--In ternary alloy joint thicknessand alloy composition formed after reflow versus the indium layerthickness on the solder column for different reflow temperatures rangingfrom 180° C. to 265° C.

FIG. 3 is a graph showing the Pb--Sn--In ternary alloy joint thicknessformed after reflow versus the reflow temperature at various indiumlayer thickness on the solder column structure. The composition of theformed ternary alloy is also shown.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1-3 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

Any suitable solder of the prior art may be used to make a solder columnstructure of the invention. Preferably the solder is a solder commonlytermed a binary solder and comprises, by weight, about 1-5% tin,preferably about 3% tin, and the balance essentially lead including theusual impurities. Pure lead is preferably used. A specific alloy whichis preferred because of its demonstrated effectiveness is a soldercontaining about 3% by weight tin and the balance essentially lead.

The solder column structure of the invention may be formed using anysuitable forming method. A preferred method employs melting (casting) apre-measured or pre-weighed amount of solder into mask cavities andcooling the mixture to form the solder in solid form. The reflow(melting) temperature is about 350°-360° C. for the 3% tin solder.

In general, one method of forming a tall C-4 solder column structure isby the use of graphite boats or forms which feature perforatedcylindrical holes with a height equal to the desired height of thesolder column. The holes are aligned with the C-4 BLM metallization padson the chip or wafer. Solder wires of the proper Pb--Sn composition,i.e., Pb-3% Sn, are inserted into the holes and reflowed at about 350°C., thus forming a cast column whose base has reacted with the BLM thinmetal film pads during reflow, forming a strong bond between the padsand the base of the column through the formation of intermetallicsbetween Sn and the reacting metals of the BLM. There is no indium atthis joint and the solder column structure is bonded (attached) to thepads of the substrate.

Another approach comprises injecting molten solder (IMS process) intothe cavities/holes as shown in U.S. Pat. No. 5,244,143 rather thanemploying preformed wires. Both of these approaches are adaptable ordirectly usable for the casting of tall C-4's as proposed herein.

After reflowing and forming the solder column structure on thesubstrate, the indium or other metal alloying layer is added. Anythickness layer may be used which leads to the formation of a ductileternary alloy joint, e.g., Pb--Sn--In. During chip joining, the jointformation occurs at a temperature lower than that of the reflowtemperature used to make the solder structure bonded to the BLM pad. Thethickness of indium or other metal layer can vary up to 1 mil or higher,e.g., 0.5 to 2 mil for most applications, independent of the size of theinterconnection in terms of column height and diameter. Indium melts atabout 155° C. and chip-joining can be easily effected at reflowtemperatures ranging from for example 200° to 250° C. with excellentwettability. At chip-joining, lead and tin start to dissolve (melt)together into the molten indium. The dissolution process continues untilthe lead-tin level of the ternary alloy reaches the level at which theternary alloy solution solidifies. It is an important feature of oneaspect of the invention that the ratio of lead to tin (Pb/Sn) in theternary is substantially the same as that in the Pb--Sn binary. This hasbeen found when the tin level of the solder is below about 3%. When thePb--Sn solder column dissolves into the molten indium, it does souniformly at the Sn--Pb ratio of the solder, since it is hypothesizedthat upsetting that ratio would require Sn or Pb diffusion within thesolid alloy, which is infinitesimally slow relative to the dissolutionkinetics. The solidification temperature, therefore, determines thecomposition of the joining ternary alloy. Given that the binary solderalloy, i.e., Pb-3% Sn, dissolves (melts) at a fixed compositional ratio,the resulting ternary alloy at solidification has the same compositionat a given chip-joining temperature regardless of the thickness of theindium layer (which is typically much smaller than the height of thebinary solder column). If the indium layer is made thicker, theresulting ternary joint will be consequently thicker in directproportion. Thus, one can make thinner or thicker ternary joints byvarying the thickness of the indium layer, which can be made byevaporation, plating or application as a paste to the solder column.

To alter the composition of the ternary joint, the chip joiningtemperature must be altered. To remelt the joint, therefore, thetemperature must just exceed the temperature at which the chip wasjoined. The mechanical and chemical properties of the ternary alloysthus formed are therefore in a practical range of temperatures demandedby the process. Consequently, one can adjust the joining temperature toallow remelting at some convenient temperature or to prevent remeltingby a subsequent process step. This latitude is important in terms ofcost and ability to overcome hierarchical temperature sequences duringthe assembly fabrication process. Again, this is possible because theternary alloys formed are preferably a one solid solution, though ofvarying composition, and consistent with respect to properties relevantto fatigue and corrosion.

Referring to FIGS. 1A-1E, the formation of a solder column structure ofthe invention and use of the column to join two substrates is shown. InFIG. 1A, a first substrate generally indicated as 10 comprises a firstbase substrate 11 and a silicon oxide layer 12. Substrate metallization13 has a BLM pad 14 formed thereon. In FIG. 1B, a mold 15 having anopening 15a is positioned on the surface of layer 12, with the opening15a corresponding and registering with BLM pad 14. A 3% Sn weightpercent Pb--Sn solder alloy wire 16 is shown inserted in opening 15a.

The structure of FIG. 1B is reflowed melting the solder alloy wire 16partially filling the mold opening 15a with a reflowed solder column 17as shown in FIG. 1C. A layer of indium 18 is shown filling the opening15a to the top of mold 15 forming the solder column structure 19 of theinvention. The column structure 19 is bonded to BLM 14 as shown in FIG.1D. The height of column structure 19 is the sum of the height A ofreflowed solder column 17 and height B of indium layer 18. A in thisexample is about 5 mils and B is about 0.7 mils.

The solder column structure 19 shown in FIG. 1D is used to join the pad23 of a second substrate 20 by reflowing at 230° C. forming a joinedassembly 25. After reflow, the interconnection 24 has a solder columnheight A' shown as 22 and is now about 4.25 mils high and the ternaryalloy joint height B' shown as 21 is about 1.2 mils high. The soldercolumn 22 is still the original binary solder having a composition of 3%by weight Sn-balance lead. The ternary solder joint 21 has acomposition, by weight, of about 40% indium, 1.8% tin and the balancelead.

FIG. 2 shows a graph of the reflow formed Pb--Sn--In ternary alloy jointthickness at solidification versus the indium layer thickness fordifferent reflow temperatures ranging from 180° to 265° C. The ternaryalloy composition differs for each temperature, i.e. Pb-1.05Sn-65In at180° C. to Pb-2.25Sn-25In at 265° C. independent of the initial indiumlayer thickness. For example, as shown in FIGS. 1A-1E a 0.7 mil layer ofindium produces a 1.2 mil (30 micrometers) thick joint of aPb-1.8Sn-38In ternary alloy at 230° C. If the reflow temperature israised to 265° C., the ternary alloy joint thickness is about 1.94 milsor 48.5 micrometers at a composition of Pb-2.25Sn-25In. At a 230° C.reflow temperature, the ternary alloy composition remains fixed as thethickness of the indium layer is varied, which changes the ternary jointthickness proportionally. Thus, at the same reflow temperature,increasing the indium from 0.7 mil to 1.2 mils leads to a proportionalincrease in the ternary layer from 1.2 to 2 mils.

FIG. 3 shows the ternary alloy joint thickness at solidification inmicrometers (microns) versus the chip-joining temperature for differentindium thicknesses ranging from 10 to 25 micrometers. The graph alsoshows the Sn and In concentrations associated with each temperature intwo separate scales.

It is an important feature of the invention that the composition andthickness of the ternary alloy joint can be controlled at anytemperature by the proper choice of the indium thickness and reflowtemperature, according to the alloy thermodynamics. The ternary alloysolidifies at the reflow temperature when it reaches the propercomposition as more Pb--Sn alloy dissolves (melts) into the liquid(molten) metal phase. The reflow temperature dictates the compositionwhile the initial indium thickness determines the final thickness of theternary alloy at the joints. Therefore, an infinite number of choicescan be made by varying the reflow temperature and the indium thicknessindependently. The important thing to be emphasized for the ternaryalloy is that for the example shown, it is basically a single-phasealloy for levels of tin below 3% independent of the indiumconcentration; that is, if the Pb--Sn alloy making up the column is asingle phase, the indium will not alter that and will form a singlephase with the solder. The advantage of this system is that themechanical properties (foremost fatigue properties) of the ternary jointare at least as good as those of the binary (Pb-3Sn) alloy assuring thatthe strain during thermal cycle fatigue is evenly distributed thuspreventing high local strains due to the abrupt variations in themechanical properties and heterogeneities, which results, for example,when a Pb--Sn eutectic alloy is joined with high-lead Pb--Sn binaries.The binary alloy strength, stiffness, and hardness generally increaseswith tin content, and its ductility degrades when alloyed with thehigh-tin Pb--Sn eutectic alloy. On the other hand, the composition rangeof the Pb--Sn--In ternary alloy joints for a wide range of chip-joiningtemperatures will have fatigue and corrosion resistance at least as goodas the Pb-3Sn column. Further electromigration problems are not aconcern as with the solder joints. In fact, this problem is notparticularly relevant since there is no In in the BLM side of theassembly which is most vulnerable to electromigration due to the smallsize of the via where the electron current (electron wind) flows fromthe BLM into the solder and solder depletion occurs due to fluxdivergence at the boundary. But the BLM side does not contain In andtherefore electromigration is minimal. In any event, there would stillbe no concern since the indium-bearing ternary alloy is substantially asresistant to electromigration as the Pb-3Sn binary. Note that with thisternary alloy system of the invention substrates can be joined at anytemperature above the indium melting point which satisfies the thicknessand composition desired, and a subsequent temperature cycle peaking at alower temperature will not remelt the prior joint and will not impairits key properties.

The flexibility and consistency of the ternary solder joint ishypothesized to be the result of a single phase which assureshomogeneity and isotropic properties throughout the ranges ofcompositions that are covered in the process and the temperatures thatprevail in the fabrication of the assembly. The proposed ternary alloysystem and associated process will achieve fatigue enhancement as highas a factor of ten to twenty-five (depending on the aspect ratio) fortall solder columns as a result of the ductile properties of the ternaryalloy formed at the joints independent of the joining temperature andfinal composition.

It has been found that using the solder of the invention providessoldered joints such as C-4 joints to have an extended fatigue lifecompared to solder joints made with solder columns of the prior art. Thefatigue enhancement in terms of fatigue life at the tolerance fail levelis generally more than two to three times the prior art alloys which arecommonly used to make solder joints.

Another benefit of the invention is that the extended fatigue life ofthe solder joints is attained both with and without the need for modulehermeticity and/or with and without an epoxy underfill. Sealing of themodule and/or the use of epoxy underfill is generally used to extend thelife of the component and the use of the ternary solder with itsenhanced fatigue life obviates the need for such techniques to increasethe life of the electronic component. If such techniques are employedusing the solder of the invention even more enhanced component lifeproperties would be achieved.

It is also contemplated herein that other metals such as antimony may beused to form a ternary alloy. Also, multiple metal layers or mixed metallayers may be used to form alloys with the melting solder column. Whilethe formation of a single phase bonding alloy (ternary, quaternary,etc.) is preferred, multi-phase alloys may also be formed and employedas the bonding joint.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

Thus, having described the invention, what is claimed is:
 1. A solderstructure in the form of a column which, when used to bond a first andsecond electronic substrates together, forms an enhanced fatigueresistant solder bond the solder column structure comprising a soldercomprising less than about 3% by weight tin, the balance lead, thecolumn having at one end the solder forming the column and at the otherfree end thereof a layer of metal thereon which metal forms a singlephase ternary alloy with the solder when the solder column is reflowed,the solder column being attached at the solder end to one of a pluralityof pads or other bonding sites on the first electronic substrate to bejoined, and, when the solder column structure is to be reflowed to jointhe solder column to the second substrate to be joined, the metal layerfree end portion of the solder column is positioned proximate thecorresponding pad or other bonding site on the second substrate with theproviso that the ratio of lead to tin in the ternary alloy is about thesame as in the solder.
 2. The solder structure of claim 1 wherein thelayer of metal is indium.
 3. The solder structure of claim 2 wherein thesolder comprises, by weight, about 1-5% tin and the balance essentiallylead.
 4. The solder structure of claim 3 wherein the solder containsabout 3% by weight tin.
 5. The solder structure of claim 3 wherein theheight of the solder structure is up to about 10 mils.
 6. A solderstructure in the form of a column which, when used to bond electronicsubstrates together, forms an enhanced fatigue resistant solder bond thesolder structure comprising a solder column attached at one end to oneof a plurality of pads or other bonding sites on one of the electronicsubstrates to be joined and having at the other end a layer of indiumwhich forms a single phase ternary alloy with the solder when the soldercolumn is reflowed to join the solder column to a corresponding pad orother bonding side on the other substrate, the solder comprising, byweight, about 1-3% tin and the balance essentially lead and the solderstructure having a solder column height up to about 10 mils and anindium layer height up to about 1.5 mils with proviso that the ratio oflead to tin in the ternary alloy is about the same as in the solder. 7.A solder structure in the form of a column which, when used to bondelectronic substrates together, forms an enhanced fatigue resistantsolder bond the solder structure comprising a solder column attached atone end to one of a plurality of pads or other bonding sites on one ofthe electronic substrates to be joined and having at the other end alayer of indium which forms a single phase ternary alloy with the solderwhen the solder column is reflowed to join the solder column to acorresponding pad or other bonding side on the other substrate, thesolder comprising, by weight, about 1-3% tin and the balance essentiallylead and the solder structure having a solder column height of about9-15 mils and an indium layer height of about 0.5 to 2 mils with theproviso that the ratio of lead to tin in the ternary alloy is about thesame as in the solder.
 8. The solder structure of claim 7 wherein theratio of the height of the solder structure to the diameter of thesolder structure is up to about 3.